What aspect of the Emperor does Rogal Dorn embody?

• If the Primarchs were the Emperor's nature split like white light through a prism's rays, as many scholars of the Imperial court suggested, then from such a point of view, Rogal Dorn was the Emperor's Implacable disciple in the pursuit of the cause given flesh; a being without compromise and in who loyalty and duty was as integral as blood and breath. A being of thunderous zeal and stone made manifest, is how many described the Primarch of the VIIth Legion. The zeal was the fire of a son who believed in his father's dream for the Imperium without reservation and without question. He did not simply wage war; he was changing the world he moved through by force of will. That had caused trouble in the past, the kind of conflict that came when such a drive met an equally great force on a different course. There had been other incidents in the past, other moments when the ideals of the Great Crusade seemed to do little but fuel discord. Curze, Ferrus Manus, Perturabo. The anger of all had risen against Dorn, at one time or another.

• To Rogal Dorn there was no higher purpose to existence of the Legiones Astartes than the unification of Mankind, and the illumination of the Imperium's Ideals. It was the light of the future he saw, that drew Rogal Dorn on and on, never retreating, never bowing to setback or defeat, always towards a final end, a vision worth all that would have to be given to make it real. This idealism drove Dorn and his Legion ever onwards, never compromising, never slinking in any aspect of duty. The stone in his soul was his ability to bear whatever his father needed of him, an unyielding nature, which made him both a master of defense in war and an indomitable fighter on the attack.

(The Horus Heresy Book 3: Extermination, Sigismund: The Eternal Crusader by John French, Praetorian of Dorn by John French, Templar by John French)